The present invention relates to a rotating electrical machine control system that includes a power supply converting portion for boosting an output of a DC (direct current) power source, and that controls a rotating electrical machine for driving a vehicle. Moreover, the present invention relates to a vehicle drive system including the rotating electrical machine control system.
In recent years, automobiles having a smaller environmental burden than automobiles driven by an internal combustion engine in which fossil fuel is combusted have been proposed. Examples of such automobiles include electric cars driven by a motor as a rotating electrical machine, and hybrid cars driven by an internal combustion engine and a motor. Motors which are mounted on the electric cars and the hybrid cars are expected to provide a desirable torque suitable for passenger car driving over a wide speed range (rotational-speed range).
A motor as a rotating electrical machine (a motor and a generator) operates based on a principle of generating a force (torque) by a magnetic field and a current. However, during rotation of the motor, the force is applied in the magnetic field, causing a so-called counter electromotive force. The counter electromotive force is generated in a direction that prevents the current flow which generates the torque. This reduces the current flowing in the magnetic field for rotating the motor, thereby reducing the force (torque). The counter electromotive force increases as the rotational speed of the motor increases. Thus, when the rotational speed reaches a certain value, a generated current reaches a driving current due to the counter electromotive force, and the motor cannot be controlled. Therefore, “field-weakening control” is performed in order to reduce the force of the field magnet generating the magnetic field to suppress generation of the counter electromotive force. However, since the field-weakening control reduces the force of the field magnet, the intensity of the magnetic field is also reduced, resulting in reduction of the maximum torque obtained. Moreover, reduction in efficiency due to increased losses has also been pointed out.
In view of this problem, Japanese Patent Application Publication No. JP-A-H10-66383 (Patent Document 1) proposes a technique of boosting a battery voltage for supplying driving electric power to a motor in order to shift the rotational speed at which shifting to the field-weakening control is performed to a higher rotational speed. According to this technique, the battery voltage is boosted by a booster circuit (converter) according to the position of a target operating point of the motor which is set according to the torque and the rotational speed. This enables a region where the field-weakening control is performed to be shifted to a higher output side (a higher torque side and a higher rotational speed side). In the example described in Patent Document 1, a region where normal field control (typically, maximum torque control) is performed without performing the field-weakening control is increased stepwise by setting a plurality of stages of boosted voltage values.
Japanese Patent Application Publication No. JP-A-2005-210779 (Patent Document 2) discloses a control technique of preventing an excessive current from flowing into such a booster circuit (power supply unit). According to this technique, a torque command of a rotating electrical machine is limited so that the sum of power consumption of the rotating electrical machine and a variation in electric power accumulated in a smoothing capacitor does not exceed the output limitation electric power of the booster circuit.